If you search in this forum or AnswerHub for “D3D device being lost” you will see several reports spread among several engine releases and even several games already shipped (Fortnite included) even in Unreal Engine 3. From time to time, users report this, being this happening during new engine releases or sometimes just when hotfixes land. It is very difficult to replicate, but its meaning is that for some reason the device stops working, resets itself, which makes the driver to report it for some time as being lost, but since operations inside the engine were referencing handlers which are not matching the card’s state (since it reset itself) it won’t recover from the error and will stop working. So, the only way possible to fix this is to understand why the card went into this issue for the first place, which might be a software issue or a hardware issue, since you said you didn’t change anything in your system (no overclock whatsoever or drivers) we can assume that the current load from the engine might making this issue to happen often.
Checking the reports, some which I gave some advice, lead people to try things, some handled it working after completely uninstalling drivers and reinstalling them (suggested to use DDU software to remove drivers), others removing any system overclock or even underclocking GPU or System speeds (voltages can also cause the same) fixed it, but many just could not discover the issue at all, which gives no choice unless try some other suggestions which is replacing computer parts until find the culprit. My honest advice is to check those posts, because there are just a lot of people working fine with the engine, and for this release yours is the first post about this.